Following the fabrication of thin semiconductor wafers, which typically have a diameter of 200 millimeters to 300 millimeters and a normal thickness from 725 microns to 775 microns, the wafers frequently are supported on a horizontal platform during observations or measurements of the characteristics of the wafers. These characteristics include top surface nanotopography.
Nanotopography measurements typically occur in a clean room with sophisticated metrology equipment under carefully controlled conditions. In order to ensure that the data being gathered from the detailed features which are measured on the wafer are independent of external contamination and peripheral influences, a chuck or chuck platform having a carefully polished planar surface typically is mounted to a stage which moves to facilitate measurement of different areas of the wafer. The design and construction of the chuck or chuck platform, especially the material and contact points which engage the reverse or underside of the wafer, can have a direct impact on the data gathered as the top surface is being measured. The materials from which such chucks are made are hard materials which are harder than the wafer undergoing the nanotopography measurements. Materials which have been used are: stainless steel, anodized aluminum, ceramics and various types of glass. These chucks or chuck platforms have precisely engineered flat upper surfaces, and frequently these surfaces are provided with vacuum points and channels.
The disadvantage to flat chucks made of a hard material like ceramic or stainless steel is that if the wafer is not perfectly planar, the wafer may conform to the shape of the chuck, causing the top surface of the wafer to then take on the surface characteristics of the chuck. For example, if there is a warp or a ripple in the wafer configuration, the sheer weight of the wafer itself on the upper surface of the chuck may cause the wafer to flatten out onto the chuck surface; so that the warp or ripple will not be detected by the nanotopography measurements.
Since the wafer tends to conform to the shape of a hard chuck, the top surface of the wafer then takes on similar characteristics, as described above. For example, if there is a hole or depression in the chuck, such as may be provided for air removal or vacuum holding of the wafer, then the wafer undergoes a slight corresponding deflection which is reflected as a valley or low region on the top surface of the wafer undergoing investigation. Conversely, a high spot on the chuck is seen as a peak or taller area on the top surface of the wafer. The sensitivity of these extremely thin semiconductor wafers (as mentioned above, having a normal thickness of 725 to 775 microns) is such that even sub-micron particles on the surface of the chuck may provide sufficient of the wafer to cause a “print through” of the particle on the upper surface of the wafer. This then results in erroneous measurements for nanotopography investigations.
Even though the measurements, such as nanotopography measurements, are conducted in a clean room, the high number of wafers which are transferred onto and removed from chucks in the course of a given period of time leads to the possibility of sub-micron particle contamination of the top of the chuck, thereby distorting the data which is obtained from the top surface wafer measurements.
Another type of chuck which has been used is a low-contact (pin) chuck which holds the wafer at only a few points. A disadvantage to low-contact chucks is that the points “print through” or cause high points or mountains to appear on the upper surface of the wafer under investigation opposite the contact pins holding the wafer.
A different type of chuck which has been used with nanotopography is a chuck with a flat upper surface, but with vacuum points and channels. The disadvantage of flat chucks made of hard material, as described above, is that the vacuum tends to pull the wafer down onto the chuck surfaces; so that the supporting surfaces between the vacuum channels tend to appear as high spots or mountains on the wafer surface undergoing measurement. Chuck designs involving removal of much of the chuck surface also result in mesas or high points on the chuck. However, as with other low contact chucks, the mesas on the surface of the hard chuck also can be seen as a print through on the top surface of the wafer undergoing measurement.
It is desirable to provide a support platform for wafers and other flat substrates not subject to the above disadvantages.